Remote metering system



E. l. GREEN ET AL REMOTE METERING SYSTEM Filed March 21, l 953 3 Sheetsheet l INVENTORS ATTORNEY May 5e 1936. E, l, GREEN E1- AL 2,039,405

REMOTE METERING SYSTEM Filed March 21, 1935 3 sheets-sheet 2 EZGIceeI/b/@Wldd 6 ATTORNEY May 5, 1936. E, GREEN p -r AL 2,039,405

REMOTE METERING SYSTEM INVENToRs I4 El Gieen L WE1/"idd ATTORNEY Patented May i, 1936 UNITED STATES PATENT OFFICE American Telephone and Telegraph Company, a corporation of New York Application March 21, 1933, Serial No. 662,003

22 claims. (o1. 177-351) This invention Telates\to arrangements for indicating at some point, by electrical translating means, the position ofone or more movable elements or the magnitude of one or more quanti- 5 ties located at a remote point.

It is the obect of the invention to provide a system adapte quantities as pressures, leve1s,`positions, flows,

` voltages, currents, watts, etc.

l0 More particularly, it is an object of this invention to provide a remote metering system in which no source of power or moving electrical [contact is necessary at the remote point where the quantityto be measured exists.

l.) In certain applications of remote'metering systems, such *asin foo" nection with gas distribution systems, oil pipe `li es land certain similar cases, a source of power or a\ moving electrical contact at the point where the quantity to be measured 0 originates might be hazardous due to the possibility of sparking. Itmight also Min certaincases be uneconomical to furnish tasource of ypower at the remote point due to the nature of the location. It is with these conditions in mind that the 5 systems about to be described have been designed. f

In accordance with one embodime t of the invention, two frequencies are transmi ted from the pointwhere the indication is to be utilized to the remote point where the quantity to be indicated exists.

frequencies are intermodulated to produce a y `third frequency which is returned to the transmitting end. The system is so designed that the magnitude of the third frequency resulting from the modulation process is proportional to the magnitude ofthe quantity to be indicated.''At

the transmitting end, the returned frequency is :selected and used to actuate a suitable indicat- 40/ing device. In another form of the invention, it

is proposed to transmit only a single frequency,

generatefo-ne of its harmonics at the remote location,v control the amplitude of this harmonic in accordancelfwith thequantity to be indicated and return this harmonic frequency to the transmitting location to produce an indication of the magnitude of ,the/distant quantity.

h It is a further object of the invention to provide a remote metering system in which no moving electrical contact or source of power is necessary at the remote ypoint and in which the amplitude relationf'between/ currents of several frequencies is used to produce an indication of 'the' quantity, the] frequency being so chosen and r utilized that the indication is substantially into the remote metering of such` At' this remote point, these two@ dependent of variations in the transmission n characteristics of the channel.

While the invention will be dened in the appended claims, the features of the invention will be more fully understood fromthe detailed exposition which follows, read in connection with the accompanying diagrams, Figures l to 7. Fig. l-a is a diagram showing one embodiment of a remotek metering system in which two frequencies are transmitted and one frequency returned. 10 Figs. l-b and l-c show other arrangements at the remote point where the quantity to be indicated s located. Fig. 2-a shows a possible embodiment of a system in which a single frequency is transmitted and one of its harmonics isre- 15v turned. Fig. 2-b shows an alternative arrangement for the distant end. Fig. 3 represents a system in which current of one frequency is transmitted and direct current returned. Fig.

4 shows a method of applying the frequencies 20 to the line in such manner that transmissions in opposite directions will cause n'o mutual interference. Figs. 5-a, 5 1);` 5-c and f5- d illustrate several methods of inserting amplifiers or repeaters in a wire line which would be suitable for use 25 with remote metering systems similar to those described herein. Fig. 6 shows an arrangement in kwhich four frequencies are sofchosen and ,used

that the indication' is substantially independent of variations in the transmission equivalent of 30 the line. Fig. 7 shows a possible way of indicating the total of several quantities existing at remote \points. i

Referring to Fig. l--a, two generators G1 and y G2 are shown which generate the'frequencies ,f1 35 and (s. 5A,combination\of filters or tuned circuits Fi transmits currents of these frequencies to the transmission line L. This: may be anyy suitable transmission channeltsuch as a pair of wires, a coaxial conductor circuit or a radio channel. The 4o transmission \ine mayl include intermediate amplifiers orfrepeq ters as indicated at R in Fig. l-a'. At the distan location currents of frequency ,f1 and f2 are\transmit`ted by the transformer TR to the modulator This modulator M may be any 45 [passive modulator such as a device using copper-` the device, therefore, varies correspondingly. 'I'he meter I1 may be a pressure gauge, lever indicator, flow meter, voltmeter, ammeter, wattmeter, or any suitable indicator. Due to this variation in the transmission efficiency of the transformer TR, the amount of the modulation products from the modulator M which is retransmitted is varied in accordance with the magnitude of the quantity measured by I1.

At the transmitting end, a filter or tuned circuit Fs selects one of the new frequencies f3 which was produced by the modulator M at the remote location. Current of this frequency I: is amplified and rectified in the amplifier-rectifier AR. the output of which is indicated by the meter I2. Due to the action of the variable magnetic coupling of the transformer TR controlled by the gauge or meter I1, variations in the magnitude of the quantity being measured are transmitted as amplitude variations of the frequency fa and these variations are indicated by the meter Iz. Therefore, the reading of the meter Iz is proportional to the magnitude of the quantity being measured. The meter may be so calibrated that the magnitude may be indicated directly in appropriate units. The output of the amplifier-rectifier AR may be used to actuate a recording device or an integrating mechanism if these features are desired in addi tion to the visual indication.

Fig. 1b shows another arrangement for the remote location. A three-winding transformer or hybrid coil TR'v is used in this case. Frequencies fr and fz are transmitted from the line L to the modulator by means of windings i I of transformer TR. The magnetic coupling between these windings and the line circuit is varied in accordance with the magnitude of the quantity measured by gauge or meter I1'. The modulator M in this case is shown as a bridge arrangement using four passive non-linear elements which may be of any i suitable type. A frequency fa which is some modulation product of frequencies f1 and f2 is applied to the line in a manner conjugate to that in which frequencies f1 and fz were taken from the line. A balancing network N is shown which balances the effect of the line in such a way that substantially no current of frequency f: is transmitted to the input of the modulator.

Fig. l-c shows still another possible arrangement of the apparatus at the remote location. In this case frequencies f1 and f2 are selected by a combination of filters or tuned circuits F4 and introduced to the modulator M" through the windings of the transformer TR. The coupling betweenv the windings of this transformer is varied in accordance with the magnitude measured by I1". 'I'he modulator may be of any suitable type. A filter or tuned circuit F5 selects frequency fa, which is a result of the modulation of frequencies f1 and f2, and retransmits current of this frequency tothe line. In this way interference between the input and output of the modulator M" is obviated.

It is possible to select for the returned frequency f: one of several frequencies depending upon the modulation process used. If the modulator is a second order device, the frequency fa may be either the sum or difference of the two impressed frequencies, that is fa=f1if2- Or in case third order modulation products are predominant, a frequency f3 might be selected which is equal to one of the frequencies added or subtracted from twice the other, that is, f1=2f1ifa or la=2fnif1- A possible modification of the above system is shown in Fig. 2--a. In this embodiment of the invention only one frequency f1 is generated by the oscillator G1. The filter F1 passes current of this frequency to the line L over which it is transmitted to the distant point where the quantity to be indicated exists. A transformer TR1 transmits current of frequency f1 to the modulator M which in this diagram is shown as a balanced modulator employing two passive non-linear elements. In this case, the second harmonic of frequency f1 is predominant and this frequency 2f1 is transmitted through the transformer TR2, to the filter or tuned circuit F2, and thence applied to the transmission channel L. The coupling of transformer TR2 is varied in accordance ,with the meter I1 thus modulating current of frequency 2f1. At the transmitting end of the line a filter or tuned circuit Fa selects this current and passes it to the ampliner-rectifier AR. The output from the amplier-rectifier is indicated on the meter Iz. The indication of meter I2 varies with the amplitude of the returned frequency 211 and thus is an indication of the magnitude of the quantity measured by the meter I1.

An alternative arrangement of the apparatus at the distant end is shown in Fig. 2-b. A filter or tuned circuit F4 and transformer TR transmit current of frequency f1 to the modulator circuit. The output of modulator M in this case is im pressed across the parallel combination of capacity C1 and inductance L1. The capacitance of the condenser C1 in this circuit is varied directly or indirectly in accordance with the magnitude of the quantity measured by I1. The variation thus produced in the impedance of combination C1 and L1 impresses a voltage on the line circuit through the filter or tuned circuit F2', which depends 0n the magnitude of said quantity. This is retransmitted and indicated at the transmitting end as in Fig. 2a.

It is, of course, possible to use in either of the systems described any suitable combination of the modulating arrangements shown in Figs. 1-a, l-b, 1'c, 2-a or 2 1), with any of the current varying devices shown in these figures.

Still another possible arrangement is shown in Fig. 3, which represents another modification of the same general idea. An oscillator G generates an alternating current which is applied to the transmission line L through condensers CR1 and CRz. Choke coils CH1 and CH2 present a high impedance to this current. At the distant location the alternating current is transmitted by transformer 'IR to the modulator M. 'I'he coupling between the windings of the transformer TR is varied in accordance with the meter or gauge I1. In this case the direct current from the modu lator M is utilized to transmit the iiuctuatlons representing the magnitude of the quantity. A low pass filter comprising condenser CH3, choke coils CH3 and CH4 and condenser CR4 passes the direct current to the line and prevents currents of other frequencies from the modulator M from flowing in the line L. The condenser CR readily passes the alternating current.

The direct current is transmitted over the line L, passed by the choke coils CH1 and CHz, and indicated by the meter I2. This indication is proportional to the magnitude of the distant quantity measured by I1.

Another possible method of separating the currents flowing in opposite directions on the line is shown in Fig. 4. Current from generator G1 is applied to the two line wires in parallel and ground in such manner as not to affect the metallic circuit, in this case by a mid-tap on the line winding of transformer TR1. At the distant end this current is taken off the line in a similar manner and applied to the modulator M". The frequency resulting from the modulation is impressed on the metallic circuit in the usual manner by transformer TR2. The magnetic coupling between the windings of this transformer is Varied by the gauge I1". At the transmitting end this current is transmitted through the repeating coil TR1, passed by the filter or tuned circuit Fa', and applied to the amplifier-rectifier AR. The meter I2" indicates the amplitude of the received current and therefore is representative of the magnitude of the distant quantity.

Figs.V 5-a, 5b, 5c and 5--d illustrate several arrangements for intermediate amplifiers or repeaters in the transmitting channel such as shown at R in Fig. 1-a. In Fig. 5--a transmission from the line L at the left to L' on the right is accomplished through transformer TR1, amplifier A2 and transformer TR2. 'Ihe transformers are of the three-winding or hybrid coil type. The output from amplifier A2 is applied to the line L in such a way that no voltage is applied to the input of amplifier A1. This is accomplished by means of the balancing network N2 associated with the hybrid coil TR2. Transmission in the opposite direction is accomplished in the same manner.

Fig. 5--b shows an arrangement employing only one amplifier and hybrid coil. The output of amplifier A3 is applied to the transformer TR; so that no voltage is induced in the input of the amplifier. In this way amplification for currents iiowing in both directions on the line is accomplished with a single amplifier so that there is no mutual interference.

Since the currents in the two directions are of different frequency, an arrangement such as that shown in Fig. 5c may be used for intermediate amplification. A combination of filters or tuned circuits passes currents fiowing from left to right from the line L to amplifier A5, and another combination of filters or tuned circuits F2 passes these frequencies to the line L'. Amplification in the opposite 'direction is accomplished by means of filters or tuned circuits F3 and F4 and amplifier A4.

Fig. 5--d shows a modification of the above system using only one amplifier for amplification in both directions. Filters or tuned circuits F5 and Fs pass the frequencies in one direction while filters F7 and Fs pass the frequencies in the opposite direction. The same amplifier is used for all the currents as shown.

An arrangement is shown in Fig. 6 in which the indication of the remote quantity is transm mitted as the amplitude ratio of currents of two different frequencies. As in the other cases no source of power or moving electrical contact is necessary at the point when the indication originates. Oscillators G1 and G2 generate currents of two frequencies f1 and f2, which are transmitted to the line through the combination of filters or tuned circuits F1. At the remote loca-n tion two filters F2 and F3 separate these frequencies. Current of frequency f2 is varied by the coupling between the two windings of transformer TR in accordance With the meter or gauge I1. Current of frequency ,f1 is transmitted to the modulator unvaried. Both frequencies are introduced to the modulator MD which may be any passive modulator, as already described.

From the output of the modulator MD two new frequencies are selected, designated as f3 and f4. These two frequencies are passed by the combination of filters or tuned circuits F4 and applied to the line L.

At the transmitting end these frequencies are separated by the filters or tuned circuits F5 and Fs. Current of frequency f4 is introduced to the amplifier-rectifier AR1 and resultant direct current is impressed on one winding 6| of the differential relay DR. 'I'he received current of the other frequency fa is passed through the variable attenuator VA, amplified and rectified in AR2 and impressed on the other winding 62 of the differential relay DR.

4Normally the armature of the differential relay DR is in a neutral position when the currents flowing in the windings 6I and 62 are equal. When a sufficient difference exists between these two currents, one contact of the differential relay will be closed. This energizes either relay 63 or relay 64. Contacts of these relays are so arranged that power is applied from a suitable source to the motor MR to cause it to rotate in either one direction or the other, depending on the operation of the differential relay DR. The motor MR, through a coupling 65, changes the position of the Variable attenuator VA in such a way that the equality of the currents in the windings SI and 62 of the differential relay is restored.

An indicator I2 is attached to the variable attenuator VA to indicate its position. The position of variable attenuator VA is a measure of the amplitude ratio of the two received currents. Since this ratio has been made dependent on the magnitude of the distant quantity, the dial I2 may be made to indicate the magnitude of this quantity in a way which will be explained.

The operation of the system shown in Fig. G will now be analyzed for a particular case, and the method of determining the frequency allocation so that the indication will be independent of the line characteristics will be illustrated. Let it beassumed that the currents generated by oscillators G1 and G2 may be expressed as sine functions, such as i=wt, where 1:2102 f being the frequency. Assume that the amplitude of each frequency is unity when transmitted. Now consider that the line loss is made up of two components,

one, denoted by a, which is constant at all frequencies, and another, denoted by b, which varies in some known manner with frequency and also may vary from time to time. Now the currents arriving at the distant point may be expressed as:

iin-abi Sin ont (1) i2=ab2 Siri wit (2) After being separate-d, current of frequency f2 is varied by the transformer TR in the ratio c. Current of frequency f1 is introduced into the modulator MD unvaried. The currents at the input of the modulator may now be expressed as:

i2'=ab2c Sill wzlf (4) Now assume that the modulator MD is of the third order type and let the output frequencies selected by filters F3 and F4 be chosen so that fs=3f1, and f4=2f2f1. Currents of these frequencies may be written:

After being transmitted back to the starting point the effect of the transmission efficiency of the line will be expressed by:

Current of frequency f3 is now attenuated by the variable attenuator VA in the ratio d and the gain of the amplifier-rectifiers ARi and ARz Will be denoted as ei and e2, respectively. The currents may now be expressed as Since the currents in the differential relay DR are maintained equal by the variable attenuator, the amplitudes of the input currents to the rectiflers ARi and ARz must also be equal. Therefore:

If now we express the above current ratio in decibels* relative to an arbitrary reference value,

*See Ilecilwl-thv name for 111e transmission unit," by W. II. Marini, Hell System Technical Journal, January i021).

we may write Equation (11) thus 20 log 1/24-80 log a-i-GO log bri-20 log bri-+20 log d-i-20 log e2:20 log 3/4+8O log a+20 log bi+40 log bz+40 log c+20 log 274-5-20 log ei (12) If we designate the ratio a as A (in db), that is, A320 log a, and similarly throughout, we may write This equation will be satised if the following relations obtain:

If now we know the manner of variation of transmission loss with frequency, we may determine the frequencies to ber used so that the condition of Equation (16) will be satisfied. As an example assume that the variable component of the loss B in db varies linearly with frequency, that is, Biff/1, etc. Then from Equation (16) Substituting these values in Equation (17) we obtain In the example assumed, therefore, the trans mitted frequencies should have the ratio 1.5 in order to satisfy Equation (16). The gain of the amplier-rectifiers ARi and AR2 may be easily adjusted to satisfy Equation (14). Therefore, Equation (l5) will hold true, that is, the indication which depends on D will correspond with the magnitude of the distant quantity, which controls C, as shown, regardless of the changes in the transmission efficiency of the line.

In case it is desirable to indicate a total of the magnitudes of several remotely located quantities, this may be readily accomplished by means of the arrangement shown in Fig. 6. Three remote metering systems similar to those described in Figs. 1, 2 or 3 are shown. each consisting of a transmitting apparatus TR, a line L, modulating apparatus MD, a meter or gauge I, receiving apparatus RC and a meter I. The magnitude of the quantity measured by I1 is indicated by the meter Ii as has already been described, the output ciu'- and 1.5

rent of the receiving apparatus RC being proportional to the magnitude of the distant quantity. In order to obtain an indication of the total of the output currents from the different receivers RC1, RC2 and RC3, these outputs are connected in parallel and the total is indicated by I4. The indication on the meter I4, therefore, is proportional to the total of the magnitudes of the distant quantities.

Any of the various known methods of indicating the total of 'overal indications may be used in conjunction with remote metering systems of the type disclosed herein. One such method has been described in the inventors copending application No. 662,002, filed March 21, 1933. It would also be easily possible to actuate an integrating device of some suitable type in connection with any oi the remote indicating systems described herein.

Although the invention has been herein described in connection with certain particular embodiments, it will be understood that many modications, both of circuit arrangement and instrumentalities employed, will be made withoui departing from the spirit or scope of the invention as set forth `in the appended claims.

What is claimed is:

1. The method of producing at a distant poinl an indication of a quantity existing at some other point which consists in varying, at( the poinl where said quantity is located and in accordancr with the magnitude thereof, the amplitude o.' current received from said other point, producing from said varied current a current of difieren' characteristics but having the same variations transmitting said current so produced to said dls tant point and utilizing the received current a.' said distant point to produce an indication of thi magnitude of said quantity.

2. The method of producing at a point an indication of a quantity existing at some othe: point which consists in transmitting electrica energy from said first mentioned point to sail second mentioned point, varying the amplitudl of the received energy at said second-mentioner point in accordance with the magnitude of sai( quantity, translating said varied energy into en ergy of different characteristics but having thl same variations, transmitting said translated en ergy to said first *mentioned point and utilizinl the energy received at said first mentioned poin to produce an indication of said quantity.

3. The method of producing at a distant poin an indication of a` quantity existing at some othe point which consists in transmitting to said las mentioned point a plurality`f frequencies, utiliz ing thesefrequencies to produce a plurality o other frequencies, varying the current amplitude of said last mentioned frequencies in accordanc with the magnitude of said quantity, transmit ting said varied currents to said distant point anl utilizing the amplitudes of said currents to pro duce an indication of said quantity.

4. The method of producing at a point an in dication of a quantity existing at some other poln which consists in producing at said first men tioned point a plurality of frequencies, transmit ting said frequencies to said second mentionel point, utilizing said frequencies to produce a plu rality of other frequencies7 varying the amplitud of at least one of said last mentioned frequencie in accordance with the magnitude of said quan tity, transmitting said last mentioned frequencie back to said first mentioned point and utilizin the amplitude of the received frequencies to pro duce an indication of said quantity.

5. The method of producing at a point an indication of -a quantity existing at some other point which consists in producing at said first mentioned point a plurality of frequencies, transmitting said frequencies to said second mentioned point, utilizing said frequencies to produce a plurality of other frequencies, varying the amplitude of at least one of said last mentioned frequencies in accordance with the magnitude of said quantity, transmitting said last mentioned frequencies back to said nrst mentioned point and utilizing the amplitudes of the received frequencies to produce an indication of said quantity, and adjusting said sets of frequencies so that said indication is substantially independent of variations in the efficiency of the transmission channel.

6. The method of producing at a point an indication of a quantity existing at some distant point which consists in producing at said first mentioned point a plurality of frequencies, transmitting said frequencies to said distant point, utilizing said frequencies to produce a plurality of other frequencies, modulating the amplitudes of said last mentioned frequencies in accordance with the magnitude of said quantity, transmitting said modulated frequencies back to said rst mentioned point over the same transmission channel, and utilizing the amplitudes of the received frequencies to produce an indication of said quantity.

7. The method of producing at a point an indication of a quantity existing at some distant point, which consists in producing at said first mentioned point two frequencies, transmitting said frequencies to said distant point, utilizing said frequencies to produce a third frequency, modulating the amplitude of said third frequency in accordance with the magnitude of said quantity, transmitting said third frequency back to said rst mentioned point and utilizing the received amplitude thereof to produce an indication of said quantity.

8. The method of producing at a point an indication of a quantity existing at some other point which consists in producing at said first mentioned point a frequency, transmitting said frequency to said second mentioned point, utilizing the received frequency to produce a second frequency, varying the amplitude of said second frequency in accordance with the magnitude of said quantity, transmitting said varied Wave back to said first mentioned point and utilizing the amplitude thereof to produce an indication of said quantity.

9. The method of producing at a point an indication of a quantity existing at some other point which consists in producing at said first mentioned point a plurality of frequencies, transmitting said frequencies4 to said second mentioned point, Varying the amplitude of said frequencies in accordance with the magnitude of said quantity, rectifying said frequencies, transmitting the resulting direct current back to said rst mentioned point, and utilizing the amplitude thereof to produce an indication of the magnitude of said quantity.

10. The method of producing at a point an indication of a quantity existing at some other point which consists in transmitting from said first mentioned point to said second mentioned point a frequency, varying the amplitude of said frequency in accordance with the magnitude of said quantity, rectifying said frequency, transmitting the resulting direct current back to said first mentioned point, and utilizing the amplitude thereof to produce an indication of the magnitude of said quantity.

11. In a remote indicating system, means at a point for varying, in accordance with the magnitude of a quantity, the amplitude of a plurality of currents received from some other point, means for transmitting said varied currents to some distant point, and means at said distant point for utilizing the received currents to produce an indication of the magnitude of said quantity.

12. In a remote indicating system, means for producing a plurality of currents of different frequencies, means for transmitting said currents to a distant point, means at said distant point for producing a plurality of currents of frequencies different from said original frequencies, means for varying the current of at least one of said latter frequencies in accordance with the magnitude of a quantity existing at said distant point, means for transmitting said last mentioned frequencies to a point Where it is desired to produce an indication of said quantity, and means for utilizing the amplitudes of the received frequencies at this point to produce an indication of the magnitude of the distant quantity.

13. In a remote indicating system, means for producing two currents of different frequencies, means for transmitting said currents to a distant point, means at said distant point for utilizing said currents to produce a third current of frequency different from said first two freuencies means for varying the current amplitude of said third frequency in accordance with the magnitude of a quantity, means for transmitting current of said third frequency to the point of origin of said first two frequencies, and means for utilizing the received amplitude of said third frequency to produce an indication of the magnitude of said quantity.

14. In a remote indicating system, means for producing a frequency, means for transmitting said frequency to some distant point, means at said distant point for producing a harmonic of said rst frequency, means for varying the amplitude of said harmonic in accordance with the magnitude of a quantity, means for retransmitting said harmonic to the point of origin of said first frequency, and means at said point for utilizing the amplitude of said harmonic to produce an indication of the magnitude of said quantity.

15. In a remote indicating system, means for producing current of a desired frequency, means for transmitting said current to a distant point,

`means for varying the amplitude of said current in accordance with the magnitude of a quantity, means for rectifying said current, means for transmitting the resulting direct current back to the point of origin of said original frequency, and means for utilizing the amplitude thereof to produce an indication of the magnitude of said quantity.

16. In a remote indicating system, means at some point for producing two currents of different frequencies, means for transmitting said currents to a distant point, means at said distant point forproducing a third frequency, said producing means requiring no source of power, means for varying the amplitude of said third frequency in accordance with the magnitude of some quantity, said varying means involving no moving electrical contacts, means for transmitting said third frequency back to the rst mentioned point, means at said rst mentioned point for utilizing the amplitude of said third frequency to produce an indication of said quantity.

17. In a remote indicating system, means at some point for producing current of a desired frequency, means for transmitting said current to a distant point, means at said distant point for producing a harmonic of said first frequency, said producing means requiring no source of power, means for varying the amplitude of said harmonic in accordance with the magnitude of some quantity, said varying means involving no moving electrical contacts, means for transmitting said harmonic back to said first mentioned point over the same channel, means at said first mentioned point for utilizing the amplitude of said harmonic to produce an indication of said quantity.

18. In a remote indicating system, means for producing two currents of different frequency, means for transmitting said currents to a distant point, means at said distant point includ ing a modulator for producing a third frequency dependent on said first and second frequencies, means for varying the amplitude of said third frequency in accordance with the magnitude of some quantity, means for applying said third frequency to the transmission channel in such a Way that no component of said third frequency is applied to the input of said modulator, means at the receiving location for utilizing the amplitude of said third frequency to produce an indi cation of the magnitude of said quantity.

19. In a remote indicating system, means for r generating at some point two currents of different frequency, means for transmitting said currents to a distant point, means at this point for utilizing said two frequencies to produce a third and a fourth frequency, the amplitude of one of which is varied in accordance with the magnitude of some quantity, means for transmitting said third and fourth frequencies over the same channel to said first mentioned point, and means at said first mentioned point for utilizing the amplitude relation between said third and fourth frequencies to produce an indication of the magnitude of said quantity.

20. In a remote indicating system, means for generating at some point two currents of different frequency, means for transmitting said currents to a distant point, means at said distantA point for utilizing said two frequencies to produce a third and a fourth frequency, the amplitude of one of which is varied in accordance with the magnitude of some quantity, means for transmitting said third and fourth frequencies over the same channel to the first mentioned point and means at said first mentioned point for utilizing the amplitude relations between said third and fourth frequencies to produce an in dication of the magnitude of said quantity, the apparatus mentioned being adjusted to bring the said four frequencies to such values that the said indication will remain substantially constant for variations in the transmission characteristics of the said channel.

2i. In a remote indicating system, means for generating at some point a plurality of frequencies, means for transmitting said frequencies to a distant point, means at said distant point for utilizing said frequencies to produce a plurality 0f other frequencies, means for varying the amplitude of at least one of said last mentioned frequencies in accordance with the magnitude of some quantity, means for transmitting said last mentioned frequencies over the same channel to said first mentioned point, and means at said first mentioned point for utilizing the amplitude relations between said llast mentioned frequencies to produce an indication of the magnitude oi said quantity.

22. In a remote indicating system, means fox generating at some point a plurality of frequencies, means for transmitting said frequencies tc a distant point, means at said distant point foi utilizing said frequencies to produce a plurality of other frequencies, means for varying the amplitude of at least one of said last mentionec frequencies in accordance with the magnitudi of some quantity, means for transmitting sai( last mentioned frequencies over the same channel to said rst mentioned point, and means al said first mentioned point for utilizing the amplitude relations between said last mentioned fre quencies to produce an indication of the magnitude of said quantity, the apparatus mentione: being adjusted to bring the said four frequencie to such values that the said indication will re main substantially constant for variations in th transmission characteristics of the said channel ESTILL I. GREEN. WARREN H. TIDD. 

